1. Field of the Invention
The invention relates to printers that print characters and images on a recording sheet by causing the capacity of pressure producing chambers to be changed by the operation of piezoelectric vibration elements in response to print signals so that ink droplets can be jet out of the corresponding nozzle openings.
2. Background
An ink jet recording head forms images on a recording sheet or the like by jetting ink droplets from the nozzle openings thereof while changing pressure in the pressure producing chambers. In order to improve image quality, small dots at a higher density must be formed on the recording sheet or the like.
Since it is difficult to adjust the quantity of ink in a single ink droplet, many ink droplet adjustment methods involve the step of jetting a plurality of ink droplets in accordance with a density specified by a print signal. However, these methods address the problem that print quality is impaired by satellites and the like.
In order to overcome such problems, e.g., Unexamined Japanese Patent Publication No. Sho. 59-133066 and U.S. Pat. No. 5,285,215 have disclosed the following method. The method involves the steps of: jetting a plurality of ink droplets, which are to form a single ink droplet, from the nozzle openings continuously; causing such plurality of ink droplets to be combined into a single ink droplet during flight; and causing the combined single ink droplet to deposit on a recording sheet. As a result of this method, gradation representation can be implemented without displacements between ink droplets caused by the travelling of the carriage.
This method will be described in more detail. A first portion of pressure change consists of a decrease in the pressure of a pressure producing chamber which increases the capacity of the pressure producing chamber. The increase in pressure is initiated at a front edge portion of a drive pulse. A second portion of pressure change consists of an increase in the pressure of the pressure producing chamber which decreases the capacity of the pressure producing chamber. The decrease in pressure is initiated at the rear edge portion of the drive pulse. This increase in the pressure of the pressure producing chamber causes an ink droplet to be jet out of the pressure producing chamber. Further, the velocity of the second portion of pressure change is proportional to the slope of the rear edge portion of the drive pulse.
The above-described method involves the following steps. First, a voltage is applied to a converter means so that the converter means contracts along the length thereof to thereby increase the capacity of the pressure producing chamber and allow the pressure producing chamber to generate negative pressure therein. As a result of this operation, ink from an ink supply reservoir is caused to flow into the pressure producing chamber so that the pressure producing chamber is filled with the ink.
In the second step, the supply of voltage to the converter means is terminated. As a result, the capacity of the pressure producing chamber is rapidly decreased to thereby allow the ink droplet to be jet out of an orifice.
In the third step, a plurality of continuous drive pulses are supplied to the converter means. In the third step, at least one of the drive pulses is applied within a time period from the end of the last drive pulse to the timing at which the tail end portion continuous to the last ink droplet jet out by the pulse moves away from the orifice, and at least one drive pulse has a larger rear edge portion slope than the last drive pulse.
The fourth step is to repetitively operate the converter means at sequentially decreasing drive cycles, which in turn produces a plurality of predetermined continuous pressure changes in the pressure producing chamber. This operation allows a plurality of predetermined ink droplets, whose velocities are continuously sequentially increasing, to be jet out from the orifices within such a time period as to allow the ink droplets to be combined together during flight.
However, the cycle of the converter means must be sequentially changed, and this operation complicates clock signal design and thus makes print control difficult. The invention has been made in view of such problems.